Four Swords is one of those good Zelda ideas that should never completely die. Every iteration so far (not counting The Minish Cap, but you don't exactly see "Four Swords" in its title, do you?) has been kind of half-baked, like Nintendo wasn't really trying that much. What we need is another Four Swords game with some real time and dedication put into it to ensure high replayability and quality.
A 3D Four Swords would be incredible, I'd think. Honestly, anything you could do in the original FS games, you should be able to easily replicate in 3D. Nintendo should go for a different visual style, too, since the Wind Waker inspiration was kind of done to death. Imagine the competition between the four Links in 3D that lets us see subtle body language and facial expressions, all trying to be the one to save the maiden first. Not to mention Four Swords has a wealth of good ideas it could try, so Nintendo should definitely tap into this potential in the future.